Since then, I have never seen an actual report of a webmaster being emailed. That changed today. A DigitalPoint Forums thread has a webmaster who has said he received this email. Here is a copy of the email he received from "Google Search Quality [noreply@google.com]":

Dear site owner or webmaster of xxxxxxxxx.com,

We recently discovered that some of your pages can cause users to be infected with malicious software. We have begun showing a warning page to users who visit these pages by clicking a search result on Google.com. Below are some example URLs on your site which can cause users to be infected (space inserted to prevent accidental clicking in case your mail client auto-links URLs):

Here is a link to a sample warning page: http://www.google.com/interstitial?u...x.com/pm/xxxxx

We strongly encourage you to investigate this immediately to protect your visitors. Although some sites intentionally distribute malicious software, in many cases the webmaster is unaware because:

1) the site was compromised
2) the site doesn't monitor for malicious user-contributed content
3) the site displays content from an ad network that has a malicious advertiser

If your site was compromised, it's important to not only remove the malicious (and usually hidden) content from your pages, but to also identify and fix the vulnerability. We suggest contacting your hosting provider if you are unsure of how to proceed. StopBadware also has a resource page for securing compromised sites: http://www.stopbadware.org/home/security

Once you've secured your site, you can request that the warning be removed by visiting http://www.stopbadware.org/home/review and requesting a review. StopBadware and Google will jointly investigate and reply to you with our findings. If your site is no longer harmful to users, we will remove the warning.

Sincerely,
Google Search Quality Team

Yes, we have seen these warnings come up as searchers but never as a webmaster. This is a very nice added service from Google. Hopefully, it won't be abused.